feat: decouple read FeedId from inbound MailboxId

Separate the two feed identities so the public read URL never reveals the
inbound address and vice-versa:

- FeedId becomes an opaque high-entropy token (read id + KV key); MailboxId
  (noun.noun.NN) owns the inbound address and the untrusted-input boundary
  via MailboxId.parse. They map only through the inbound:<mailbox> secondary
  index, resolved solely at reception.
- inbound index lifecycle is owned by FeedRepository: written by save/saveConfig,
  dropped by removeFromList(Bulk) — symmetric, never mirrored by hand (removes the
  manual delete in feed-service + the cron loop, and a silent empty-catch).
- Feed.mailboxId exposes a MailboxId VO (symmetry with Feed.id); the
  mailbox@domain shape lives on MailboxId.emailAddress(domain).
- Distinguish mailbox_unknown (no feed claims the address) from feed_not_found
  (dangling index) for observability; both forwardable, both 404.
- Drop the redundant EmailParser.extractMailbox pass-through so MailboxId.parse
  is the single parse boundary.

Docs (README/INSTALL/CLAUDE.md/landing) and tests updated; 439 tests green,
tsc clean, build dry-run OK.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/domain/value-objects/feed-id.test.ts

@@ -1,36 +1,27 @@
 import { describe, it, expect } from "vitest";
 import { FeedId } from "./feed-id";
 
-describe("FeedId.parse", () => {
-  it("extracts the feed id from an inbound address", () => {
-    expect(FeedId.parse("river.castle.42@example.com")?.value).toBe(
-      "river.castle.42",
-    );
-  });
-
-  it("preserves the original casing of the local part", () => {
-    expect(FeedId.parse("River.Castle.42@example.com")?.value).toBe(
-      "River.Castle.42",
-    );
-  });
-
-  it("rejects malformed feed ids", () => {
-    expect(FeedId.parse("user@example.com")).toBeNull();
-    expect(FeedId.parse("notanemail")).toBeNull();
-    expect(FeedId.parse("river.castle.4@example.com")).toBeNull();
-    expect(FeedId.parse("river.castle.123@example.com")).toBeNull();
-  });
-});
-
 describe("FeedId.generate", () => {
-  it("produces the noun.noun.NN format", () => {
+  it("produces an opaque base64url token", () => {
     for (let i = 0; i < 50; i++) {
-      expect(FeedId.generate().value).toMatch(/^[a-z]+\.[a-z]+\.\d{2}$/);
+      expect(FeedId.generate().value).toMatch(/^[A-Za-z0-9_-]{22}$/);
     }
   });
 
-  it("round-trips through parse from an address", () => {
-    const id = FeedId.generate();
-    expect(FeedId.parse(`${id.value}@example.com`)?.value).toBe(id.value);
+  it("is unguessable: 50 ids are all distinct", () => {
+    const ids = new Set(
+      Array.from({ length: 50 }, () => FeedId.generate().value),
+    );
+    expect(ids.size).toBe(50);
+  });
+
+  it("does not produce the legacy noun.noun.NN format", () => {
+    expect(FeedId.generate().value).not.toMatch(/^[a-z]+\.[a-z]+\.\d{2}$/);
+  });
+});
+
+describe("FeedId.unchecked", () => {
+  it("wraps a value without validation", () => {
+    expect(FeedId.unchecked("anything").value).toBe("anything");
   });
 });